The modern banjo includes a plurality of models, which includes the four-string version (plectrum and tenor), five-string versions, and even six-string versions. The sound produced by a banjo is characterized by poorly sustained notes that are preferably deep and rich. As a musical instrument, the banjo is unusually amenable to modifications. For example, a player may tighten or change the head, which is the stretched membrane disposed beneath the strings to amplify the sound of the strings when played. Additionally, heavier or lighter bridges may be used, or the strings may be changed from nylon to steel for example.
A resonator banjo, which is the type normally used for bluegrass music, has a body that includes a head, a tone ring and a rim. Typically, the head is a membrane that is stretched across the generally circular tone ring, which in turn rests upon and is connected to the rim, which is normally made of wood. On an external circumference of the body, there is a supporting device called a “resonator flange,” which promotes centering of the rim and suspension of the rim at least partially inside the resonator. The resonator of a resonator banjo consists of a generally rounded back section and a circular side wall.
A cavity within the body of the banjo and the resonator is called an “air chamber.” The air chamber communicates to an outside of the banjo at a bottom portion of the rim. In most cases, the bottom portion of the rim generally does not contact the resonator. The volume of the air chamber is determinative of some of the frequency and amplitude characteristics of the banjo.
Conventional methods of adjusting the volume of the air chamber within the banjo included moving the resonator with respect to the rim and/or the head of the banjo. For example, moving the resonator farther away from the rim and/or the head increases the volume of the air chamber. Over the years, banjo players and luthiers that specialize in the banjo have determined that each individual instrument has an optimum air chamber volume, which generally must be determined according to the individual components of each individual banjo because each instrument may have different vibrational characteristics. The difficulty posed by this method is that in many cases, the resonator mounting loses stability, which can cause mechanical problems with the banjo.